In an optical communication system, optical amplifiers are provided at each preset distance in the optical transmission path to realize long-distance optical transmission. For example, in the optical transmission path across the Pacific Ocean, several tens to several hundreds of optical amplifiers are provided. For example, one of the optical amplifiers is an optical fiber doped with rare-earth ions.
In the meantime, along with increasingly wide use of the Internet, capacity of data transmitted via networks drastically increases, and it is required that the transmission capacity of optical communication systems be increased further. One of the techniques for increasing the transmission capacity of an optical transmission system is wavelength division multiplexing (WDM), and a WDM transmission system has been put into practical use.
In the WDM transmission system, plural signals having different wavelengths are multiplexed, and transmitted through one optical fiber. Hence, in the WDM transmission system, it is required that the optical amplifier be able to amplify the plural signals at one time. Meanwhile, the so-called ADM (Add Drop Multiplexer) device, which increases and decreases the number of frequencies of individual light signals, and in which power of the light signals transmitted by the optical fiber is not constant, has been put into practical use. Even when the number of frequencies of signals varies, it is expected that power of the output light at a specified frequency can be controlled to be constant by controlling the gain of optical amplifiers. This control is referred to as AGC (Automatic Gain Control).
However, in the WDM transmission system, when the distance between optical amplifiers is long, signal attenuation in the transmission path increases; thus amplifiers of high gains are required, but this causes various noises.
The reasons for the noises in optical amplifiers probably include external noise, noise from optical elements, and ASE (Amplified Spontaneous Emission) noise due to natural light emission.
FIG. 1 shows signal spectra from an optical amplifier using an erbium doped fiber (EDF).
As illustrated in FIG. 1, comparing the spectra before and after amplification, it is found that there is a spectrum in a broad band added to the amplified signal after amplification. This additional broad band spectrum is called ASE noise, which is generated when a portion of natural light emission is coupled with the basic mode of an optical fiber, and is amplified due to induced emission.
When the number of frequencies of a light signal is small, the portion of these noises relative to the total power of the signal is not negligible any longer when executing AGC.
For example, Japanese Laid Open Patent Application No. 9-200145 and Japanese Laid Open Patent Application No. 7-221737 (below, referred to as “reference 1” and “reference 2”) disclose techniques of controlling an amplification gain to be constant in optical amplifiers capable of fast response. In reference 1, a fiber is inserted between an input light monitor and an optical amplifier to delay a light signal. In reference 2, adjustment light, which is used for making adjustment, is input to cancel out variation of an input light, thereby, obtaining a constant gain.
The technique disclosed in reference 1 provides a margin in control time, thus enabling fast response, but the excessive optical fiber causes degradation of performance, and the optical amplifier becomes large.
The technique disclosed in reference 2 is able to narrow the range of the power of the input light to be adjusted, and thus facilitates realization of fast response, but it becomes necessary to control the adjustment light at a high speed, and power consumption and temperature increase due to additional output of the adjustment light.
In AGC of the related art, both the noise component and the signal component are amplified if the influence of noise on the input light is large, and this causes variation of the power of the output light.
During operations of increasing or decreasing the number of frequencies of light signals, the variation of power of output light caused by noise is not a severe problem, because this variation of the power is small compared to the variation of the power of the output light caused by the operations of increasing or decreasing the number of frequencies. However, in a steady state, that is, without the operations of increasing or decreasing the number of frequencies, the variation of the power of the output light caused by noise is dominant, and it constitutes the variation of the power of the output light. Therefore, it is required to reduce amplification of the noise.
To reduce amplification of the noise in input signals, it is sufficient to just reduce a feedback control coefficient. However, if the feedback control coefficient is maintained to be small constantly, tracking due to AGC is delayed during operations of increasing or decreasing the number of frequencies of light signals, and the variation of the power of the output light increases. To solve this problem, if it is set to switch the feedback control coefficient during operations of increasing or decreasing the number of light signal frequencies and in the steady state, it is necessary to devise a method of determining the switching, and to reduce the variation of the power of the output light caused by changing of the coefficients during the switching. In the related art, these factors are not taken into consideration.
In addition, concerning the ASE noise, the influence thereof increases when the number of light signal frequencies is small. If assuming the power of the input light to an optical amplifier is In, the power of the output light from the optical amplifier is Out, and an object gain is G, the gain error (GE) of AGC control can be calculated by using the following formula.GE=In×G−Out,
Here, considering an ASE noise ASEnoise is input to the optical amplifier, then
                    GE        =                              (                          In              +              ASEnoise                        )                    ×          G          ⁢                      -                    ⁢          Out                                        =                              (                          1              +                              ASEnoise                ⁢                                  /                                ⁢                In                                      )                    ×          In          ×          G          ⁢                      -                    ⁢                      Out            .                              
If the power of the input light In decreases due to decrease of the number of light signal frequencies, the gain error (GE) of the AGC control increases, and this causes output ripples (variation of the power). For this reason, it is necessary to monitor the power of the input light and make compensations. However, compensations for the AGC control are not performed in the related art.